The invention relates to a sheet feeder which feeds sheets, received in a stack within a sheet cassette, one by one to a copying machine, a printing machine or the like (hereafter collectively referred to as a copying machine) by the action of a feed roller which oscillates in response to a feed signal, and more particularly, to such sheet feeder which is adapted to produce a given amount of flexure in the sheet as it is fed from the cassette and before it reaches register rollers.
A conventional sheet feeder as used in a copying machine is constructed in a manner as illustrated in FIG. 1, for example. Specifically, a copying machine includes a loading station 10 which receives a sheet cassette 1 having a number of sheets 2 disposed therein in a stack. It is to be understood that the sheets 2 in the stack are upwardly urged by a bottom plate 3, as formed by a leaf spring, which is disposed in the bottom within the cassette 1, whereby an uppermost one of the sheets 2 has its upper surface engaged and retained by separation claws 8 while a feed roller 6 moving toward an opening 1a, formed in the top panel of the cassette 1 toward the front end thereof, feeds the uppermost sheet out of the cassette 1. As shown, the feed roller 6 is rotatably mounted on a pin 5 secured to the free end of an arm 4 which is in turn mounted on a shaft 7 in a rockable manner. A drive mechanism, not shown, causes the roller to rotate in a direction to feed the sheet. The sheet 2 delivered out of the cassette 1 by the action of the feed roller 6 is guided by a pair of guide plates 9a, 9b toward a nip between register rollers 11.
In a conventional sheet feeder constructed in a manner as mentioned above, a feed signal applied causes the arm 4 to move down until the feed roller 6 engages the upper surface of the sheet 2. Thereupon, the force of friction acting between the rotating roller 6 and the sheet 2, combined with the action of the separation claws 8, causes the uppermost sheet 2 to be driven forward past the separation claws 8, whereby such sheet moves along the guide plates 9a, 9b until its leading end reaches the register rollers 11 which then remain stationary. During the time the arm 4 remains at its lower position for a given time interval, the roller 6 continues to feed the sheet, whereby the uppermost sheet 2 which has its leading end held in abutment against the register rollers 11 will be flexed upward, as shown in phantom line in FIGS. 1 and 2, on the lower guide plate 9a. When the arm 4 returns to its upper position after the given time has passed since the feed signal has been initially applied, the feed roller 6 moves away from the sheet surface, whereby the feeding operation of the sheet 2 is terminated. Subsequently, when a drive signal is applied to the register rollers 11 to cause rotation thereof during the time the roller 6 continues its feeding operation, the sheet is fed into the interior of the copying machine.
In the conventional sheet feeder constructed in the manner mentioned above, it will be seen that there is a great difference in the amount of flexure formed in the sheet being delivered when the leading end thereof is held in abutment against the register rollers 11, between when the cassette 1 is full of fresh sheets 2 and when the stack contains a few last sheets, as shown in FIG. 3. The greater the depth of the cassette 1, the greater will be the magnitude of such difference. Such flexure is indicated at 2a and is effective to build up a force to drive the sheet 2 forward, which may be utilized when feeding the sheet 2 into the copying machine through the register rollers 11. Accordingly, if the amount of flexure is excessively low, the sheet will be urged against the register rollers 11 with a reduced force, thereby causing a likelihood that a lag in the feed operation may result. Conversely, if the amount of flexure is excessively high, the flexure 2a will be formed at two locations, namely, in the form of a peak and valley as shown in FIG. 4. In this instance, the force to drive the sheet 2 forward will be lost, degrading the mating relationship with the register rollers 11 to increase the likelihood of producing a lag in the feed operation.
It will be seen that such a varying amount of flexure produced in the sheet 2 results from the fact that the downward movement of the feed roller 6 is always initiated at a given timing independently from the number of sheets left in the stack within the cassette 1. Referring to FIG. 5 which illustrates the point of contact of the roller 6 with the sheet 2, it will be noted that the location of the upper surface of the uppermost one of the sheets 2 in the stack when the cassette 2 is full of sheets is indicated by a line A while the location of the surface of the uppermost one of sheet 2 in the stack when only a few sheets are left within the cassette is indicated by a line B. It will be apparent from FIG. 5 that a point of contact a between the roller 6 and the sheet 2 for the full stack is displaced from a point of contact b for the nearly exhausted stack by a vertical distance .delta.. Accordingly, the sheet which begins to be fed from the point b will be fed by a length which is less than the corresponding length of the sheet which begins to be fed from the point a, by an amount corresponding to a time interval required for the movement through the vertical distance .delta.. As a consequence, the amount of flexure formed in the sheet 2 after the leading end of the sheet 2 abuts against the register rollers 11 is reduced. The magnitude of the vertical distance .delta. increases with an increasing sheet capacity of the stack, causing an increasing difference in the amount of flexure between the full stack and the nearly exhausted stack, giving rise to the likelihood that a lag in the feed operation by the register rollers 11 may result.